Resorcinol-glutaraldehyde resin as an accelerator for curing phenol-formaldehyde resins

ABSTRACT

The invention relates to the use of a resorcinol-glutaraldehyde reaction product as an accelerator for thermosetting phenolic resin and to a method of making the accelerator. Further, the invention relates to a resin blend comprising phenol-formaldehyde and the accelerator and a process for making cellulosic board, oriented strandboard and plywood wherein the binder for the cellulosic board, oriented strandboard and plywood in its uncured form comprises the resin blend of the invention. In another embodiment the accelerator for thermosetting resin comprises resorcinol and glutaraldehyde.

This is a division of application Ser. No. 07/991,208 filed Dec. 15,1992, now U.S. Pat. No. 5,364,902 issued Nov. 15, 1994.

FIELD OF THE INVENTION

This invention relates to an improved phenolic resin that is useful asan adhesive binder for the manufacture of hot pressed wood compositessuch as hardboard, fiberboard, strandboard, wood panels, particleboard,plywood, and the like.

BACKGROUND OF THE INVENTION

Phenolic resins for the wood industry have been refined for the pasttwenty years to the current "state of the art" and as such, perform to awell defined standard. The recent development of isocyanate-basedadhesives has provided wood adhesives that are somewhat faster curingthan phenolics and that are, in some cases, replacing phenolics due totheir faster cure rates, even though they have some negative health andeconomic aspects.

However, phenol-formaldehyde resins remain widely used as adhesives andbinders in many wood products, including wood products such as plywood,particleboard, fiberboard, hardboard and oriented strandboard. Theproductivity of most mills manufacturing wood products using liquidphenol-formaldehyde resole (PF) binders is limited by the cure speed ofthe binder in the hot press. This is because of the inherently slowthermal cure of these products, compared to other commonly used binders,and because of the need to eliminate moisture from the system duringcuring.

Several methods have been used to speed up the cure rate of PF resinbinders.

There are the methods in which various kinds of alkali metal hydroxides,water-soluble alkali metal weak acid salts, or water-insolublemultivalent metal carbonates are added to the PF resin in order toaccelerate its curing.

In order to speed up the cure rate (i.e, cut down the cure time), curingagents like alkylene carbonate have been utilized. U.S. Pat. No.4,961,795 to Detlefsen et al describes such a curing agent.Unfortunately, mixing this type of curing agent with aphenol-formaldehyde resin usually results in a mixture that has a verylimited "pot life" for process manipulation.

It was the intention of the inventors to develop an additive that willaccelerate the curing of phenol-formaldehyde resin and provide asynergistic effect by improving bond quality of the manufactured woodproducts bonded with the resin. Further, it was the intention of theinventors that binder made by mixing the additive with thephenol-formaldehyde resin have a reasonable "pot life."

SUMMARY OF THE INVENTION

It has been discovered that the addition of small amounts ofresorcinol-glutaraldehyde reaction product to phenol-formaldehyde resinwill accelerate subsequent heat cure of the resin. Theresorcinol-glutaraldehyde (RGL) is preferably in resin form.

However, generation of the RGL reaction product to serve as anaccelerator for curing PF resin, can be done in situ by the directmixing of a PF resin with appropriate amounts of resorcinol andgultaraldehyde prior to spraying the binder onto wood products or thelike. Further, the resorcinol-glutaraldehyde resin of the invention hasadhesive qualities that allow it to be used "as is" or in combinationwith other suitable resins to bind cellulosic components and the like.

In one embodiment, the invention is a blend of phenol-formaldehyde resinand resorcinol-glutaraldehyde resin. In another embodiment the inventionis an adhesive composition made by adding components with desiredproperties to the accelerated blend of the invention. The addedcomponents may include for instance fillers such as wood flour or thelike. Further, the invention relates to hot pressed wood composites madeusing the inventive adhesive.

According to one embodiment of the invention, the accelerator isprepared by allowing glutaraldehyde to react with resorcinol to producea resorcinol-glutaraldehyde resin using a molar ratio of glutaraldehydeto resorcinol in the range from about 0.5:1 to about 2.5:1. Resin blendsof the invention may contain about 1 to about 25 parts ofresorcinol-glutaraldehyde resin and about 99 to about 75 parts of anaqueous solution of an alkaline phenol-formaldehyde resole resin havingsolids content of at least about 40% by weight. The blends are useful asbinders in the manufacture of wood panels using a conventional spraying,hot-pressing process. Strandboards produced according to the inventionhad stronger bond strength and better durability achieved at relativelyshorter press cycles, than the control strandboard in which the adhesivewas a conventional PF resole resin, without an accelerator additive.

In another embodiment, the invention relates to cellulosic board and toa process for making cellulosic board wherein the binder for thecellulosic board in its uncured form comprises a resin blend accordingto the invention. Further, the invention relates to oriented strandboardand to a process for making the strandboard wherein the binder for thestrandboard in its uncured form comprises a PF resin blended with anaccelerator additive, according to the invention. Further, the inventionrelates to plywood and to a process for making the plywood wherein thebinder for the plies of the plywood in its uncured form comprises aresin PF blended with an accelerator additive according to theinvention.

In further embodiments, the invention relates to binders consistingessentially of RGL resins, formulated adhesive binders that comprise RGLresins and to cellulosic products that contain any of these binders.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the use of a mixture or blend of anaccelerator and a PF resin, referred to herein as an accelerated PFresin, for the bonding of lignocellulosic materials such as in themanufacture of plywood, hardboard, particleboard, fiberboard, orientedstrandboard and the like. Phenol-formaldehyde resole resins areconventionally utilized in the manufacture of structural wood product,i.e., for the bonding of lignocellulosic materials. Thephenol-formaldehyde resin may be used as is or may be extended by mixingthe resin with flour or other suitable fillers. The as is PF resins aretypically used for composition panels while the extended resins are usedfor plywood.

Process For Preparation of the Accelerator

The accelerator of the invention, resorcinolglutaraldehyde resin, isprepared by allowing glutaraldehyde to react with resorcinol.

Typically, a 25% glutaraldehyde aqueous solution is first concentratedat 52° C.±4° C. to raise its concentration to 30% to 40%. The pH of the25% glutaraldehyde solution from the supplier is generally no more thanabout 3 but could be lower depending on its age, etc. During the processof raising the concentration of the initial glutaraldehyde solution, thepH can be adjusted to be in the range from about pH 3 to about pH 10.3.Bases such as soda ash, caustic soda, potassium hydroxide,triethanolamine, and their mixtures, may be used to adjust the pH. Thespecific base used is not critical as long as the pH is controlled. Theglutaraldehyde concentration is not necessarily limited to a range of30% and 40%; other concentrations can also be used.

Resorcinol is then introduced to react with the glutaraldehyde. Themolar ratio of glutaraldehyde to resorcinol may be varied from 0.5:1 to2.5:1. Resorcinol can be added in one full charge or in several separateincremental charges at different reaction stages. The use of 2 or 3separate incremental additions of resorcinol is preferred. The pH of thereaction mixture usually should range from 5.5 to 8.5, and is preferablyin the range of 6.0 to 7.5. The pH ranges are convenient for control ofthe reaction, but are not critical.

The reaction can be carried out from room temperature to reflux with thepreferred range being between 60° C. and 80° C. This preferredtemperature range here is for convenience in the control of thereaction. The resinous reaction product will generally have anon-volatile content of 40% to 60%, with 45% to 50% as the preferredrange, and will have a pH of 5.0 to 7.5. The pH range is determined bystorage requirements.

The Phenolic Resin

Phenolic resins that can be used in the present invention arewater-soluble thermosetting condensation products which are made byreacting one or a mixture of hydroxy aromatic compounds (phenols) withone a mixture of aldehydes using an alkaline catalyst.

Phenols used generally are phenol, cresol, and other substitutedphenols. Substituted phenols employed in the formulation of the phenolicresins include: alkyl substituted phenols, aryl-substituted phenols,cycloalkyl-substituted phenols, alkenyl-substituted phenols,alkoxy-substituted phenols, aryloxy-substituted phenols andhalogen-substituted phenols, the foregoing substitutents containing from1 to 26 and preferably from 1 to 6 carbon atoms. Specific examples ofsuitable phenols, aside from the preferred unsubstituted phenol, incude:m-cresol, p-cresol, 3.5-xylenol, 3.4-xylenol, 2.3.4-trimethyl phenol,3-ethyl phenol, 3.5 diethyl phenol, p-butyl phenol, 3.5-dibutyl phenol,p-amyl phenol, p-cyclohexyl phenol, p-octyl phenol, 3.5 dicyclohexylphenol, p-phynyl phenol, p-crotyl phenol, 3.5-dimethoxy phenol,3.4.5-trimethoxy phenol, p-ethoxy phenol, p-butoxy phenol,3-methyl-4-methoxy phenol and p-phenoxy phenol.

The aldehydes reacted with the phenol can incude any of the aldehydesheretofore employed in the formation of phenolic resins such asformaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde andbenzaldehyde. In general, the aldehydes employed have the formula R'CHOwherein R' is a hydrogen or a hydrocarbon radical of 1 to 8 carbonatoms. The most preferred aldehyde is formaldehyde. There are includedin aldehyde substances that may be reacted with the phenol formaldehydesources or donors such as formalin, para-formaldehyde,alpha-polyoxymethylene, hexamethylene tetramine, etc.

Caustic soda is the most suitable practical alkaline catalyst, butcaustic potash, barium hydroxide, mixtures of these, basic alkalinesalts such as alkali metal carbonate, and mixtures of such salts withone or more of these hydroxides, can also be included. As the aqueoussolvent, water is the most suitable, but mixed solvents which consist ofwater and water-soluble lower alcohols, water and other water-solubleorganic solvents are included. These aqueous solvents are used withsolid contents of such phenolic resins, being between about 30% and 60%,preferably between about 35% and 50%, and more preferably at least about40% solids by weight.

The molar ratio of phenol to aldehyde for the preparation ofwater-soluble thermosetting phenolic resins used in the presentinvention is generally from about 1:1.5 to 1:3, preferably from about1:1.7 to 1:2.5, and these are reacted with an alkaline catalyst in anaqueous solvent. It is preferable for the water-soluble phenolic resinfor use in the present invention to be stable for long term storage,therefore such a water-soluble phenolic resin is reacted until its freeformaldehyde content becomes less than three percent by weight,preferably less than one percent by weight, and more preferably lessthan 0.2%.

Descriptions of procedures that may be used to make suitable phenolicresins are found in the pages of Chapter 5 of Phenolic Resins by Knopand Pilato published in 1985 by Springer-Verlag, which pages areexpressly incorporated herein by reference.

Adhesive Binder Composition of the Invention

The adhesive binder composition or resin blend of the inventioncomprises a mixture of about 1 to about 25 parts of theresorcinol-glutaraldehyde resin solution (RGL) with about 99 to about 75parts of a phenol-formaldehyde resole resin solution, with the solidscontent of the RGL solution being 40% to 60%, and with the solidscontent of the resole solution being 40% to 60%.

The blending of the accelerator and the phenolic resin can beaccomplished by any suitable means. For instance, the accelerator may bemixed into a batch of phenolic resin, or mixing can take place bymerging separate streams of each of the resins.

If desirable, other components such as filler and/or extender may beadded to the phenol-formaldehyde resin before it is mixed with theaccelerator or the components may be added at any other suitable stage.Application of the final adhesive binder system to the furnish may bedone by any conventional method such as spraying, brushing, rollercoating, or curtain coating.

The resin blend of the invention can be used to make a variety oflignocellulostic products. Methods for making plywood, cellulosic board,oriented strandboard and the like are described in prior art as forinstance in U.S. Pat. Nos. 4,758,478 to Daisy et al, and 4,961,795 toDetlefsen et al., which patents are incorporated herein by reference.For example when producing a composition panel such as particle board ororiented strandboard by a mat process, wood flakes, fibers or particlesare sprayed with a solution of accelerated resin of the invention. Thesprayed pieces of wood may be passed through a forming head to make amat. Alternatively, multiple forming heads may be employed to lay downsuccessive layers of a multiple layer product. In such a case, it isfeasible to spray the wood particles as they are being fed to eachforming head with its own supply of resin formulation. Hot pressingconditions for the mat will depend upon the target thickness for theboard product as well as on the characteristics of the binder.

It is contemplated that the inventive accelerated adhesive compositionwill be particularly useful in making plywood and its use may permitcutting down on hot press time and produce plywood with a stronger bondas compared with the use of ordinary phenol formaldehyde resins.

It is further contemplated that any thermosettable phenolic resin whencombined with the accelerator of the invention will provide an adhesivethat will require less heat energy in order to cure as compared to acontrol.

The invention will be demonstrated by the following examples. In theseexamples and elsewhere throughout the specification, parts andpercentagesare by weight and temperatures are degrees Celsius unlessexpressly indicated otherwise. The term "molar ratio" refers to themolar ratio of formaldehyde to phenol unless indicated otherwise.

EXAMPLES General Comments

The 25% glutaraldehyde solution used in the examples was obtained fromPFALTZ and BAUER, Inc. of Waterbury, Conn. It had a pH of about 3.2, andwas a clear liquid characterized by a sharp odor. The resorcinol usedwas obtained from Koppers Company, Inc. of Pittsburgh, Pa. Theseresorcinol flakes had a minimum purity (dry basis) of 99.3%. The aspenstrands used were obtained from the Potlatch Corporation of Cook, Minn.They consisted of strands of aspen, pine, birch, maple and oak and hadmoisture contents in the range of from 1% to 10%. The average strandsize was targeted to be0.03"×0.5"×1.75" (thickness/width/length).

The phenol-formaldehyde resin used was a commercial liquidphenol-formaldehyde resole resin obtained from the Borden Packaging &Industrial Products, Columbus, Ohio, sold under the trademark CascophenPB-306 . This resin had a pH of about 12.6 at 25° C., a freeformaldehyde content of <0.2%, a free phenol content of <0.50%, avolatiles content of about 53% by weight at 125° C. and a solids contentof about 47% by weight.

The viscosity of each of the samples described in the examples wasmeasuredusing a Brookfiled Synchro-Letic Viscometer Model: RVF-100,Volts: 115, Serial: 57787, Frequency: 60. Each sample was placed in a400 ml. beaker and its temperature was adjusted to 25° C. A #2 spindlewas used and the speed was set at 20 rpm. The viscosity reading wastaken at one minute from the time the motor was turned on.

Measurement of the Modulus of Rupture (MOR) was made as follows. A testspecimen of 2"×14" was cut from each board. The specimen was weighedto±0.1 gram and its thickness was measured to ±0.001 inch. The thicknessmeasurement reported was an average of three different measurements,each taken at the center (i.e., 7" from each end of the specimen) and atthe 3" center points from each end of the board. The spanused was 8.0"and the load was applied at 0.5 inches/ minute. The specimen was placedon the jig with 1/2" extending past one support and broken. Thespecimenwas then turned over and was rebroken with 1/2" extending past theothersupport. Two breaks were measured on each MOR sample and the loads wererecorded to ±0.1 lb. The MOR is reported as an average (psi) calculatedaccording to the following formula: ##EQU1##

A 6-cycle test was used to evaluate the delamination and strengthretentionof specimens of the product after six moisture cycles. It wasconducted as follows. The specimen size was 2"×14" and each specimen wasweathered per APA Test Method D-5 which is described as follows.

Specimens were placed in a rack such that they remained separatedthroughout testing to insure proper drying. The racks were then placedin a pressure vessel and completely submerged in 150° F. water. A vacuumof 15 inches of mercury was drawn, maintained for 30 minutes andreleased. Specimens were then allowed to soak in the same water atatmospheric pressure for 30 minutes with no additional heating.Afterwardsthey were removed and dried for six hours at 180° F. in anover withfan-forced air circulation of 45 to 50 air changes per minute.Specimens were then returned to the pressure vessel and the vacuum-soakcycle was repeated. Following the second vacuum-soak cycle, specimenswere again placed in the oven and dried for 15 hours at 180° F. Thiscompletedtwo cycles. This process was continued for two additional daysuntil six cycles had been completed.

The conditioned specimens were then tested using the test used for theMOR measurement.

EXAMPLE 1 A. Preparation of Resorcinol-Glutaraldehyde Resin

1000 grams of the 24.88% glutaraldehyde aqueous solution (2.485 moles)werecharged at room temperature to a 2 liter round-bottom flask and thepH of the solution was adjusted to 8.2 with 4.1 grams of anhydrous sodaash. Theglutaraldehyde solution was then concentrated by distillation ata temperature of 47° C., under a vacuum of 27.1 inches of mercury,togive a yellow, milky product after 300 grams of distillate wereremoved.

182.4 grams (1.657 moles) of the resorcinol were charged to the flaskwhen the batch temperature was of the concentrated glutaraldehydesolution 39° C. After dissolution, the clear yellow reaction mixture hada pH of 7.2 and was allowed to react at 75° C.±5° C. until its viscosityreached the letter G on the Gardner-Holt viscosity scale.

The reaction mixture was then cooled down to 40° C.±2° C.,and then thecooled mixture was charged with an additional 59.7 grams (0.542 moles)of resorcinol. The product was a clear amber liquid, which had a pH of6.3, a viscosity of 80 cps (at 25° C.) and a solids content of 51.4%.

B. Preparation of Two Phenol Formaldehyde Resin-ResorcinolGlutaraldehyde Resin Blends

Resin blends for wood products manufacturing were prepared. A firstresin blend B-1 was prepared by mixing 50 grams of theresorcinol-glutaraldehyde(RGL) resin of Step A with 950 grams of acommercial phenol-formaldehyde (PF) resole resin solution, CascophenPB-306 resin from Borden. A second resin blend B-2 was made by mixing100 grams of the RGL resin of Step A with 900 grams of the same PFresole resin solution.

C. Making of Aspen Strandboards

6851 grams of aspen strands having a moisture content of 3.8% wereplaced in a rotating drum blender and were sprayed with 698 grams of thefirst resin blend B-1. Next, 1780 grams of the resin-treated strandswere taken from the drum blender and spread evenly in a 14.75"×14.75"mold to form a mat. The mat was then hot pressed at 400° F. for anamount of time in the range of from 4 to 6 minutes to produce astrandboard specimen with a target thickness of about 0.75". This wasrepeated to produce several specimens, using different press times.

Similarly, other strandboard specimens were produced in generally thesame way as just described, but using 695 grams of the second resinblend B-2 instead of using the first resin blend B-1. Several controlstrandboard specimens were likewise made by spraying aspen strands with701 grams of the commercial PF resin instead of using resin blends B-1or B-2.

The bond strength of each strandboard specimen was then measured, andthe results are tabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                        Summary of the Averaged Bond Data                                                        Press   Internal Modulus                                                                              Modulus of                                            Cycle   Bond     of     Rupture After                              Resin      Time    Strength Rupture                                                                              6-cycle Test                               Binder     (min)   (psi)    (psi)  (psi)                                      ______________________________________                                        PF         6.00    66       3203   2017                                       Control    5.50    59       3583   1804                                                  5.00    45       3087   1581                                                  4.75    44       2213   1687                                       Resin Blend B-1,                                                                         5.50    66       3971   1999                                       Ratio of 5 Parts                                                                         5.00    51       3972   2291                                       RGL to 95  4.50    49       3230   2126                                       Parts PF   4.00    45       2562   1516                                       Resin Blend B-2                                                                          5.50    65       4158   2504                                       Ratio of 10 Parts                                                                        5.00    66       4117   2590                                       RGL to 90  4.50    56       3859   2874                                       Parts PF   4.00    49       3482   2082                                       ______________________________________                                    

Conclusions

The bond strength data shown in Table 1 demonstrate the acceleratingeffectthat the resorcinol-glutaraldehyde resin has on thephenol-formaldehyde resin cure speed and the improved bond quality ofthe wood products. The results observed when using Resin Blend B-2 areparticularly dramatic withrespect to the MOR values.

The minimum press cycle time shown in Table 1 for each resin binder wastheshortest cycle which did not show delamination upon pressure release.For instance, acceptable PF control strandboard (i.e. one that did notdelaminate upon pressure release) required a press time of at least 4.75minutes, whereas acceptable strandboards were made with the first resinblend B-1 and with the second resin blend B-2 using a press time of only4.00 minutes. These data demonstrate that acceptable strandboard can bemade more quickly and can be made using less energy than is the casewith the control strandboard.

Further, the data show the acceptable strandboard of the invention cannot only be made quicker and with less energy but also that thestrandboard ofthe invention has a greater bond strength and a highermodulus of rupture than control strandboard, when the least press timeis used that avoids delamination upon the release of pressure.

EXAMPLE 2 A. Preparation of Resorcinol-Glutaraldehyde Resin

1000 grams of the 24.88% glutaraldehyde solution (2.4850 moles) werecharged at room temperature to a 2-liter round-bottom flask and the pHof the solution was adjusted to 8.13 with 3.5 grams of anhydrous sodaash. The glutaraldehyde solution was then concentrated by distillationat a temperature of 48° C.-49° C. and under a vacuum of 27 inchesofmercury to give a yellow, milky product after 300 grams of distillatewere removed.

182.4 grams (1.657 moles) of resorcinol were charged to the flask whenthe batch temperature was 39° C. After dissolution, the clear and yellowreaction mixture had a pH of 7.07 and was allowed to react at 75° C.±5°C. until its viscosity reached the letter G of the Gardner-Holtviscosity scale.

The reaction mixture was then cooled down to 40° C.±2° C. and then thecooled mixture was charged with an additional 59.7 grams (0.5422 moles)of resorcinol. The product was a clear amber liquid which had a pH of6.08, a viscosity of 74 cps (at 25° C.), and a solids content of 50.77%.

B. Preparation of Two Phenol Formaldehyde Resin-ResorcinolGlutaraldehyde Resin Blends

Resin blends for wood products manufacturing were prepared. A resinblend B-3 was prepared by mixing 100 grams of theresorcinol-glutaraldehyde (RGL) resin of Step A with 900 grams of acommercial phenol-formaldehyde (PF) resole resin solution, CascophenPB-306 resin from Borden. Another resin blend B-4 was prepared by mixing150 grams of the RGL resin of Step A with 850 grams of the PF resoleresin solution.

C. Making of Aspen Strandboards

6809 grams of aspen strands having a moisture content of 2.7% wereplaced in a rotating drum blender and were sprayed with 713 grams ofresin blend B-3. Next, 1766 grams of resin-treated strands were takenfrom the drum blender and spread evenly in a 14.75"×14.75" mold to forma mat. Themat was then hot pressed at 400° F. for an amount of time inthe range from 3.75 to 6.00 minutes to produce a strandboard specimenwith a target thickness of 0.75". This process was repeated to produceseveral specimens, using different press times.

Similarly, strandboards were produced in generally the same way as justdescribed, but using resin blend B-4 instead of resin blend B-3. Severalcontrol strandboard specimens were likewise made by spraying aspenstrandswith 720 grams of the PF resin instead of using resin blends B-3or B-4.

The bond strength of each strandboard specimen was measured, and resultstabulated in Table 2.

                  TABLE 2                                                         ______________________________________                                        Summary of the Averaged Bond Data                                                        Press   Internal Modulus                                                                              Modulus of                                            Cycle   Bond     of     Rupture After                              Resin      Time    Strength Rupture                                                                              6-cycle Test                               Binder     (min)   (psi)    (psi)  (psi)                                      ______________________________________                                        PF         6.00    56       4090   1816                                       Control    5.50    57       3646   2182                                                  5.00    48       3504   2003                                                  4.50    49       3675   2133                                       Resin Blend B-4                                                                          5.50    64       3975   2106                                       of 15 Parts                                                                              5.00    51       3773   2085                                       RGL & 85   4.50    51       3911   2099                                       Parts PF   4.00    45       3243   2240                                       Resin Blend B-3                                                                          5.00    57       3756   2219                                       of 10 Parts                                                                              4.50    56       3781   2214                                       RGL & 90   4.00    49       3280   2226                                       Parts PF   3.75    40       3657   2100                                       ______________________________________                                    

Conclusions

This set of data reaffirms the accelerating effect of the addedresorcinol-glutaraldehyde resin on phenol-formaldehyde resin cure. Onceagain, the minimum press cycle time shown in Table 2 for each resinbinderwas the shortest cycle which did not show delamination uponpressure release.

The data indicate that the bonding strength of a PF resin adhesive isenhanced by the addition of resorcinol-glutaraldehyde resin. The datasuggest that there may be an optimum amount of RGL for any given PFresin.

GENERAL

The formulation of the phenol-formaldehyde resin depends to some extenton the end use for which it is intended and the formulation in turnaffects the curing behavior of the resin and thus the result obtainedfrom the curing agent and the accelerator (RGL). The formulationstypically used for composition panel such as oriented strandboard have asolids content of between about 50 and 60 weight percent and are free ofany extender or filler. On the other hand, the formulations typicallyused for plywood arecooked to a solids content of between about 40 and48 weight percent and then combined with a filler or extender such aswheat flour or the solid by-product obtained from the production offurfural alcohol from corn cobs.

The accelerated resin of the invention can be applied to the furnishwith any form of conventional equipment currently in use. Such equipmentincludes spray nozzles, atomizing wheels, roll coaters, curtain coatersand foam applicators. The key to successful use, however lies inobtaininga relatively short time interval between mixing of theaccelerator and the resin and application of the mixture to the furnish.This is because the viscosity of prior art accelerated resin begins toincrease and may resultin loss of solubility in water after mixing.However, the accelerated resins of the invention are quite stable forseveral hours to several daysand are suitable for use in existingequipment.

When neat resins are mixed with prior art accelerator, as for anoriented strandboard application, the mixture is usually too thick tohandle within30-60 minutes or less. The exact time depends upon thenature of the resin,the nature of the accelerator, and the proportionsof the two components. When dealing with filled adhesives, as forplywood, more pot life is available than with unfilled resins, usuallyabout 60-180 minutes. This istrue both because the filled adhesives,normally mixtures of resin, water, sodium hydroxide and fillers orextenders, do not increase as rapidly in viscosity as the neat resins,and because plywood application equipment can handle higher viscosities.Despite the greater pot life of the plywoodmixes, the rapid viscosityincrease obtained when mixed in normal mixing eqiupment, i.e., gluemixer and glue storage tank, would be considered limiting. For thesereasons, in-line mixing just prior to application to the furnish is thepreferred means of introducing the accelerator. In-linemixing isparticularly advantageous in the use of the accelerator with unfilledphenolic resin and greatly facilitates use of the agent with filledresins.

Although there is no reason why the resins of the invention cannot bein-line mixed, the longer "pot life" of the resins of the inventionallowsthe manufacturer to use more conventional mixing and glue storageequipment.

The choice of raw material for the cellulosic component is based mainlyon availability and cost. As is common in boardmaking manufacturingoperations, the wood from which particles are produced may be in theform of logs that are unsuitable for conversion into lumber or plywoodbecause they are too small, too crooked or too knotty, or the like. Whensuch logsare reduced to small particle form, defects are screened out.

The invention is useful in the production of board that is made fromhomogeneous cellulose material or from mixtures of different kinds ofsuchmaterial. A board may be made, for example, completely from woodparticles,or completely from wood flakes, or from fibers, planershavings or the like, or from mixtures of these. Similarly, a board maybe formed with multiple layers, with fine surface flakes and a core ofcoarse flakes, or it may have a coarse-flaked core with an overlay offibers on each of its surfaces. Other combinations may also be produced.

Wood flakes are generally made by a machine that shaves off flakes ofthe wood in a direction such that the length of each flake is parallelto the wood grain. a normal size flake has dimensions such as 1/4 by1"with a thickness in the range from about 0.005" to about 0.075",depending upon the intended end use.

The cellulosic material may also be in the form of wood fibers. In theproduction of such fibers, wood chips are generally mechanically reducedto fiber form in an attrition mill.

The wood pieces employed in making the composite panel have someaffinity for water and a tendency to absorb it. Water entering acomposite panel tends to weaken it, may cause some swelling of surfacefibers, and increases the dimensional instability of the compositionpanel. To preventthis tendency to absorb water, a wax may be applied tothe wood pieces to provide a built-in resistance in the compositionpanel to water absorption. The wax employed may be any wax that willsuffice, for example, a crude scale wax or a microcrystalline wax. It isapplied, generally, at a rate of from about 10% by weight to about 30%by weight ofthe binder, and preferably about 20% by weight, dry solidsbasis. When expressed in terms of oven-dried furnish solids, the amountof wax is fromabout 0.3% by weight to about 3.0% by weight wax to wood.

The amount of accelerated phenol-formaldehyde resin used generally willdepend upon the characteristics required in the final product. For ahigh grade oriented strandboard, the amount of binder used may be up toabout 5% of resin solids based on dry finished board weight andgenerally may befrom about 2% to about 4%. For a good grade ofparticleboard, the amount ofresin should be sufficient to provide fromabout 3% to about 8% dry resin solids based on the weight of the furnishfor the composite panel. In a multi-layered board, a lesser amount ofresin will often be used in the core than is used for the surfacelayers.

Hot pressing conditions will depend upon the thickness of the board aswellas on the characteristics of the accelerated resin. A representativepress cycle for the production of a 3/4" thick bonded particleboardwould be about 6.5-7 minutes at a press platen temperature of about380°-420° F. The pressing time can be reduced by the presentinventionwithout loss in board quality. For instance 3/4 inch five layer orientedstrandboard typically requires a pressing time of 540 seconds. The useof about 5 weight percent of an accelerator RGL resin, in accordancewith the invention, allows reduction of the press time without loss offinal board properties.

The invention is also useful in the manufacture of plywood, a boardcomposed of multiple layers of wood veneers. The veneers are usuallyarranged so that the wood grain direction is perpendicular in adjacentveneers.

The plywood process requires straight logs cut to length, andconditioned in heated vats containing water and surfactants to increasethe heating efficiency of the vats. The heated logs are then "peeled"wherein a veneerof predetermined thickness is removed continuously untilthe log diameter is reduced to a certain point, usually 5-8 inches. Theveneer is then clipped into strips, sorted and dried to a moisturecontent of 15% or less.

After drying, the veneers are graded and assembled into plywood panels,theadhesive binder is applied to the veneers at this stage ofmanufacture.

After the adhesive is applied to the wood veneers and the panels areassembled, they are consolidated under heat and pressure. This isusually done in a steam hot press using platen temperatures of 240°-350°F. and pressures of 47-250 psi.

In producing plywood, the most critical glueline is the innermost one.Thisglueline is the most difficult to cure under present conditions.That is, often the innermost glueline is not fully cured when the othergluelines are. It is necessary, then, to apply additional hot pressingto the board to cure this glueline. One additional use of the presentinvention is thatthe accelerated resin can be applied to the innermostgluelines. The accelerated resin is then able to provide a complete cureat the innermostglueline in the same time period as it takes to cure theother gluelines.

It has been discovered that several advantages are obtained by utilizinganaccelerated resin, i.e., a resin containing the curing agent, in themanufacture of structural wood products. One advantage is that cure timecan be decreased. A second advantage is that the addition of the curingagent increases the tolerance to moisture in the system. It iscontemplated that the system may withstand more moisture with theaccelerated resins, and make possible the production of morepremium-gradepanels. It is contemplated that the thicker the board, themore effective the accelerator, and the more significant the advantages.

While the invention has been disclosed by reference to the details ofpreferred embodiments, this disclosure is intended in an illustrativerather than in a limiting sense, as it is contemplated thatmodifications will readily occur to those skilled in the art, within thespirit of the invention and the scope of the appended claims.

We claim:
 1. An adhesive consisting essentially ofresorcinolglutaraldehyde resin and water wherein the molar ratio ofglutaraldehyde to resorcinol of said resin is in the range of from about0.5:1 to about 2.5:1 and wherein the solids content of the resin in saidmixture is in the range of from about 40% to about 60% by weight.
 2. Theadhesive of claim 1 having a pH of 5.0 to 7.5.
 3. The adhesive of claim1 wherein the solids content is from about 45% to about 50%.